The field of this invention is that of installing drag reducing fairings on vertical pipes in the ocean to reduce the side load on the pipes due to ocean currents. In deepwater offshore drilling, a riser pipe of approximately 20 inch internal diameter is used for a pathway down to the well bore to allow the control of the drilling pipes and the circulating fluids. Buoyancy material is added to these pipes to offset a majority of their weight and limit the top tension required for the system. The buoyancy on a drilling riser of this type will generally be in the range of 52 inch outer diameter.
In 2.5 to 3.5 knot currents on a 5000 foot long drilling riser can incur side loadings of up to 100,000 lbs. caused by currents. These side loadings require substantial horsepower to remain above the well below for drilling while the riser pipe is connected.
During the drilling operations, a portion of the side load is taken by the vessel on the surface, and a portion of the side load is taken by the equipment on the ocean floor.
When attempts are made to retrieve the riser under these conditions, the side forces must all be taken by structures on the drilling rig at the surface. A 100,000 lb. load of the riser against the side of the rig structures will not only completely prevent the pulling of the drilling riser, but will destroy sections of the buoyancy material which impacts the rig structures.
Fairings are devices generally in the shape of an airplane wing which are pivotably mounted on a pipe such as a drilling riser. The flow around the round riser and the wing shaped trailing portion will reduce the drag on the riser by as much as 50 percent. This does not cure all of the problems, but it beneficially increases the ocean current range in which a vessel can operate.
An additional problem surrounding drilling risers is the nature of current flow down stream of the riser. In some cases it will get swirls of water called vortexes alternating on one side of the riser and then the other. In addition to the drag loads, this induces a vibration referred to as vortex induced vibration. The smooth transition from the riser pipe diameter to a fairing profile will naturally tend to reduce the potentially destructive vortex induced vibrations.
A major problem with contemporary fairing systems is that in order to be manageable, each section is about 7 feet long at a maximum. This requires a multitude of sections to be installed on a deepwater riser drilling riser. When the drilling riser is being deployed, each 7 feet, the riser must be stopped and valuable rig time allocated attaching a fairing section. In running or retrieving a drilling riser, this operation can take an additional four or five days, with ten days for a round trip. At $400,000 per day, this is as much as a $4,000,000 expense simply to attach the fairings.
As the fairings have weight and must pivot around the riser to remain down current, they must be attached to the riser in a load bearing and pivoting fashion. As they are nominally 7 feet long, this special connection must be repeated every 7 feet. This represents both the time to stop and make the connections, but also likely a modification to the buoyancy material itself to accommodate the attachment.
On a 5,000 foot drilling riser, typically only 1,000 feet of fairing would typically be installed due to the fact that the high currents tend to be near the surface. Having fairings on the upper 1,000 feet of riser will allow the operator to release from the subsea wellhead. However, in retrieving the riser the fairings must be removed. Once the first 1,000 feet of riser are retrieved, the riser will again experience the high side forces.